1. Field of the Invention
The present invention relates to a color separation apparatus and, more particularly, to a color separation apparatus that may be used in low-priced color copiers and scanners.
2. Description of Background Art
In color copiers and scanners, there are generally two methods for separating the color of an original image being copied or scanned. One method uses a color CCD (Charge Coupled Device) sensor, and the other method uses a black-and-white (hereinafter referred to as "b/w") CCD sensor and a color separation apparatus. Color CCD sensors, however, are expensive and therefore significantly raise the component cost of color copiers or scanners that incorporate them. Accordingly, in lower-priced color copiers and scanners, b/w CCD sensors and a color separation apparatus are usually adopted for color separation.
A color separation system using a conventional b/w CCD sensor and a color separation apparatus can be further classified based upon whether a method of using R, G and B (red, green, blue) light sources, each having a different spectral distribution, or a method of using a single white light source and a color filter or a prism is used. An apparatus that employs a method that uses a prism, however, exhibits a drawback in that its structure is complicated and requires 3-line CCDs and image-forming optical systems, all of which increases the component cost of the apparatus. An apparatus that employs the method of using R, G an B light sources also exhibits a drawback in that high-speed scanning is hindered by persistence time of each light source as they are turned on and off in succession.
In an apparatus that employs a single white light source and a color filter, there are generally two methods for separating color. The first method involves arranging the filter in fixed positions, and the second method involves driving the filter to be rotated or ascended/descended in front of the sensor. However, the second method involving driving the filter requires an additional apparatus or component, e.g., a motor, for driving the filter.
To overcome these drawbacks, two different color separating methods using a fixed color filter have been proposed. The first method involves installing the color filter at a transparent glass portion where documents are loaded, as disclosed in Japanese Patent, Publication No. 60-127862, and the second method involves using a 3-line CCD and triple-layered reflection filters, as disclosed in U.S. Pat. No. 4,709,144.
The method disclosed in Japanese Patent Publication No. 60-127862 is depicted in FIG. 1. As seen in FIG. 1, a surface of a glass 2 where a document 3 is placed is coated with a strip filter 4. Upon illumination of the document 3 by light 1, an area of the document 3 in the reading section is selectively transmitted through the strip filter 4 so that color is separated, reflected by mirror 5, refocused by lens 6 and received by CCD sensor 7. However, as shown in FIG. 2, since sets of R, G & B color strip filters are arranged in a single line in a main scanning direction, resolution lowers to 1/3 or below compared with other conventional color separation apparatus, and high resolution scanning cannot be easily achieved.
The method disclosed in U.S. Pat. No. 4,709,144 is depicted in FIG. 3. Referring to FIG. 3, a color separation system comprises a trichromatic photosensor 9 (or 3-line CCD sensor) and reflection-type filters 10a and 10b (trichromatic beamsplitter) having glass plates and dichroic coatings R, B and G, that are suitable for high resolution scanning. In this method, however, light passing through a lens 8 is reflected from reflection-type filters 10a and 10b, respectively, and is thereafter incident on the 3-line CCD or trichromatic photosensor. Inevitably, there is a loss of energy as the light is made to pass through filters and glass plates, and this loss of energy may be compensated by increasing the power of the light source to obtain a desired final output. From the standpoint of physical and mechanical construction, since each color-separated light should be image-formed respectively on the 3-line CCD or trichromatic photosensor, and because the linear array of photosensors are disposed at predetermined intervals, exact and precise arrangement of the reflection-type filters is required.